Biphasic current stimulation is widely used in electrical stimulation of neurons. This constant current form of stimulation has an advantage that the stimulation is largely unaffected by changes in electrode impedance which occur at the interface between the signal conductors and the tissue.
The voltage required to stimulate an electrode is the product of the stimulation current and the electrode impedance in a similar way to Ohms law, which states that the voltage across a resistance is equal to the product of resistance and current. In the case of a biological stimulating electrode, the impedance is not a pure resistance but has capacitive or capacitance-like properties. Electrode impedance varies from electrode to electrode and over time.
For constant-current stimulation using rectangular biphasic pulses, the so-called impedance is defined as the voltage at a stimulation electrode at the end of the first phase of a stimulation pulse divided by the electrode current. While Ohms law applies strictly to pure resistance, the same principle is applied to calculate the voltage required to obtain a particular stimulation current.
A disadvantage of using constant current to generate the biphasic stimulation signals is that the signal generation circuitry does not always generate sufficient voltage in order to deliver the desired amount of charge to the tissue. FIG. 1 is a graph of voltage required at the signal generation circuit versus percentage of electrodes operating within voltage compliance for adults and children using aural prostheses that employ biphasic current stimulation. FIG. 1 shows that for some situations, a larger voltage is required in order to deliver the necessary charge than for other situations. Additionally, it is evident from FIG. 1 that, for much of the time, the maximum voltage level is not required in order to achieve charge delivery compliance and so a lower voltage would suffice.
The provision of a higher voltage (i.e. around 10 volts) to cater for the relatively small number of occasions that it is needed results in a far higher power consumption than is actually delivered to the electrodes, with the rest being absorbed in the current source. This power consumption has a significant effect on battery life for prostheses employing biphasic current stimulation.
It is desired to address or ameliorate one or more shortcomings or disadvantages associated with prior techniques for delivery of biphasic stimulation to excitable tissue, such as nerves and/or muscles or to at least provide a useful alternative thereto.